The present invention relates to ferromagnetic thin film memories, and more particularly, to ferromagnetic thin film memory cells based on multiple magnetic structures.
Digital memories are used extensively in digital systems of many kinds including computers and computer system components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, typically thin film materials. These may be ferromagnetic thin film memories which may provide access to information stored therein by either inductive sensing to determine the magnetization state, or by magnetoresistive sensing for such determination. Such ferromagnetic thin film memories may be provided on the surface of a monolithic integrated circuit to provide for convenient electrical interconnections between the memory cell and the memory operating circuitry.
Ferromagnetic thin film memory cells are usually made very small and packed very closely together to achieve a significant density of stored bits, particularly when provided on a surface in a monolithic integrated circuit. The magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetizing fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory film is provided. Such an arrangement provides a significant "flux closure" to thereby confine the magnetic fields arising in a cell to affecting primarily just that cell. This is considerably enhanced by choosing the separating material and the ferromagnetic memory film to be sufficiently thin.
Often such a digital memory is constructed by having a number of memory cell storage structures (or bit structures if a single structure per bit) connected in series at junctures to one another in an end-to-end fashion. A series of current straps, or word lines, are often provided in an orthogonal layout to the series of connected storage structures so that a strap crosses each of the structures between the junctures. In a magnetoresistive memory, such straps or word lines are used both in the entering of and the sensing of information in the bit structures. This can be done by using currents in the word lines for setting, or for determining the existing, magnetization state of storage structures in the memory.
However, with respect to magnetic fields generated by word lines over a storage structure there is no "flux closure." This is because the word line is over the top of a bit structure so that there is no closed magnetic path for magnetic fields in the structure around the word line. The result is that very large demagnetizing fields can occur in a bit structure both for entering information and for sensing information in that structure. Such fields can seriously disrupt operation of the memory.
These demagnetizing fields can be reduced by properly configuring such bit structures, providing a narrowing geometry towards the ends of the structures where they come to and under the junctures. A narrowed portion of magnetic material may extend entirely through the storage structure juncture.
With such magnetic structure arrangements, the resultant magnetization of bit or storage structures becomes more stable giving better defined alternative magnetization states for storing digital information. In those memory cell structures in which such information is to be extracted therefrom by use of the magnetoresistive properties to assess which of these magnetization states occurs in the ferromagnetic memory films, some sort of sensing current is applied through the storage structures.
Storage structures formed as bit structures, being in one of two alternate magnetization states typified by having the magnetization vector point in one of two opposite directions more or less along the easy axis, will have correspondingly different resistances at least during a reading operation depending on which magnetization state such structure is in. The sense current provided through the structure will then lead to different voltage drops across the structure depending on its magnetization state thereby providing the information as to the state it is in. Alternatively, a sensing voltage could be applied to such bit structures with resulting current differences because of the resistance differences indicating which magnetization state is present in a bit structure.
The signal information thus obtained will be relatively small because the change in resistance as fraction of the resistance of the entire bit structure is relatively small. Furthermore, the energization source to provide the sensing current or voltage will have an electrical noise component supplied therewith which, along with noise generated in the bit structure itself, will make the small signal containing the digital state information relatively difficult to discern out of the total electrical response obtained from a bit structure. Thus, an arrangement to provide an increase in such a signal and to reduce electrical noise would be desirable.